Laura Guertin: Collecting Data: Icthysticks and Otoliths, June 21, 2023

NOAA Teacher at Sea

Laura Guertin

Aboard NOAA Ship Oscar Dyson

June 10 – June 22, 2023

Mission: 2023 Summer Acoustic-Trawl Survey of Walleye Pollock in the Gulf of Alaska

Geographic Area of Cruise: Islands of Four Mountains area, to Shumagin Islands area
Location (10:45AM (Alaska Time), June 21): 55o 29.7525′ N, 156o 44.7276′ W

Data from 10:45AM (Alaska Time), June 21, 2023
Air Temperature: 8.4 oC
Water Temperature (mid-hull): 8.2oC
Wind Speed: 8 knots
Wind Direction: 20 degrees
Course Over Ground (COG): 76 degrees
Speed Over Ground (SOG): 11 knots

Date: June 21, 2023

Once the echo sounder has shown us the position of an aggregation of Alaska walleye pollock (we hope they are pollock and not some other fish species), we lower the trawl net and see what we can catch. This is where the trawl sonar and CamTrawl (see previous blog post) come in handy to give us an idea of what is going into the net. It’s an amazing coordination of effort between the acoustics lab (who decides where to trawl), the bridge for navigation, and the deck crew for setting/retrieving the haul.

We aim for trawling at the mid-water level, where the pollock are typically found. Pacific Ocean perch (POP, or rockfish) can also be found in the mid-water level in the Gulf of Alaska, especially just off the shelf break. Bottom trawls can yield pollock and other fish (e.g., POP and other rockfish species, various species of flatfish).

Once the trawl net has been brought back on board, the catch is emptied into a bin called a table. There is a door on the side of the table that opens into the fish lab. Once the table door opens, the fish spill into the laboratory where they travel down a conveyor belt for the initial sorting. Our target species is the pollock. We weigh everything that ends up onto the sorting table, either in bulk (by species) or individually.

  • pollock moving along belt
  • pile of dead rockfish
  • dead squid on the deck of a ship

A subset of around 250 pollock are set aside to collect length data. The length of these of each individual pollock are measured on an Ichthystick. This is another invention by Rick Towler and Kresimir Williams (remember the CamTrawl? (see previous blog post)). As described in their article An inexpensive millimeter-accuracy electronic length measuring board, these NOAA scientists describe using magnetic measuring technology that, to millimeter resolution, takes a measurement when you placed a magnet on a sensor that runs the length of the board. For our pollock measurements, we were looking to record the fork length, and a quick placement of the red magnet along the fish tail sends the data to a computer program called CLAMS (Catch Logger for Acoustic Midwater Surveys).

  • green plastic bins containing dead pollock (fish)
  • A close-up view of the end of the measuring board shows the larger sizes on the scale (marked 75, 80, 85). The board's logo reads "Ichthystick" and includes a stylized illustration of a pollock.
  • computer screen with long measuring board
  • illustration of a fish with lines showing the various lengths of measurement. Title: "Measuring Fish Length." labels: "Maximum Standard Length," "Fork Length," and "Maximum Total Length."
  • dead fish laying on measurement board
  • two people in rain gear in a laboratory taking measurements of fish

Another subset of approximately 50 pollock are set aside for additional data collection on individual specimens – length, weight, sex, maturity, and age. Otoliths (e.g., ear bones) are removed, and sometimes organs are removed and measured (ovaries for maturity development analyses, liver).

a black and white image showing pairs of otoliths from different fish species. Each otolith is white and gray in contrast with the solid black background; lighting reveals the ringed growth pattern
Otolith pairs (two per individual fish) from an assortment of Bering Sea fish species. Walleye pollock is located in the top left. Note: otolith sizes are not on a relative scale. Photo: NOAA Fisheries.

What are otoliths, and why remove them? Otoliths are ear stones, or ear bones, found in fish. To give you an idea of why we remove ear bones, let’s start by thinking about trees and corals… trees grow a new ring on their structure each year, and corals have differences in their skeletal density between the seasons (both trees and corals are also used to reconstruct past climate conditions (proxy data for paleoclimatology)). By counting the rings on trees and coral, we can calculate the age of that specimen. It turns out that fish also have a way to record their annual growth – and it occurs in their ear through Fish Otolith Chronologies.

Scientists are very interested in studying otoliths. When otolith data are combined with data on fish size, scientists are able to determine the growth rates of fish, which then combined with the survey work, helps inform annual fish stock assessment reports. We don’t do any of the otolith analyses on the ship, but we do collect the samples with a detailed label and all the corresponding data (fish length, sex, weight, location) that is sent back to the NOAA Fisheries Alaska Fisheries Science Center for analyses and entered into their Fish Otolith Collection Database.

  • Two otoliths in a person's hand
  • close-up view of two otoliths in a person's hand; the growth rings are visible
  • two gloved hands hold up a fish cut open to reveal the otoliths inside the head
  • person holding tweezers and placing object in glass vial

Did you know… More than 30,000 otoliths are read annually by NOAA Fisheries Alaska Fisheries Science Center scientists. So far, the Science Center has collected more than 1.1 million fish otoliths for ageing. (from NOAA Fisheries)

To learn more about the fascinating studies of otoliths and what NOAA Fisheries is doing, check out these websites:

NOAA Fisheries Age and Growth – NOAA Fisheries scientists assess the age and growth rates of fish species and populations to better monitor, assess, and manage stocks. There is also a separate site for Age and Growth Research in Alaska.

NOAA Fisheries Age and Growth Homework: Determining How Old Fish Are

NOAA Fisheries Near-Infrared Technology Identifies Fish Species From Otoliths – NOAA Fisheries scientists are developing ways to use near-infrared spectroscopy (NIRS) analysis of otoliths (fish ear stones) to provide accurate information for sustainable fisheries management faster.

If you are really curious to explore some fish otolith data, check out the Alaska Age And Growth Data Map, an interactive map displays collected specimen information from recent age and growth studies from Alaska Fisheries Science Center.

Screenshot from the Alaska Age and Growth Data Map website. On the left is a map of Alaska with lots of orange, blue, and green circles marking sampling locations. To the right are two graphs plotting lengths (y-axis) against ages (x-axis) for walleye pollock sampled in 2021. Blue circles (or box-and-whisker plots) represent samples from the Western Bering Sea and green circles represent samples from the Eastern Bering Sea.

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